The overall condition of the motorcycle is a good indicator. If it’s clean, straight, and leak-free, that’s a good sign. The more things you see that make you think, “I’ll want to fix that before long,” the sooner you should walk away. Even on a well-maintained bike of that age, you’ll want to replace the rubber brake hoses with braided stainless lines, flush and fill the brake and cooling systems, change the engine oil and filter, and check the valves if it hasn’t been done recently. Spoon on new tires if the old ones are more than four or five years old, regardless of tread wear. If the bike’s been sitting, load test the battery at your earliest opportunity and replace it if it doesn’t hold a full charge. Online forums will tell you what other specific issues to watch for with the model you’re looking at.